Hikaru no Go

Hikaru and Sai's relationship is so curious and so special because, Sai is older than Hikaru, and he's the one teaching him Go, so in a normal situation there would be a clear power dynamic between the two, but because Sai is incapable of interacting with the world without Hikaru's consent, the power imbalance is gone. Also, because of Sai's childish personality and lack of knowledge of the modern world, there are moments in which Hikaru becomes Sai's teacher and has to put up with his tantrums. It's like they take turns into becoming each other's bratty little brother. Also by the time Hikaru becomes an insei they act like twins sometimes, completely in-sync (so cute).

I think, at some point, Sai started to bleed into Hikaru. Like, he was a friend and a teacher, but Sai became a part of him. Sai was Go, and Go became Hikaru's life, so Sai became an intrinsic part of his own identity as a person. Sadly Hikaru didn't notice any of this until Sai was gone. That's also why it hurts so much, Hikaru quite literally lost a part of himself that day